This invention relates to a method for working up inactive solutions containing antimony halide catalysts and organic impurities.
Fluorochlorohydrocarbons, fluorochlorocarbons, fluorohydrocarbons and fluorocarbons are used to a large extent as solvents and refrigerants. (For simplicity, the following description will refer only to hydrocarbon compounds, but this term is to be understood as including hydrogen-free compounds, perhaps more correctly called carbon compounds. Thus, as used herein, the term "fluorochlorohydrocarbon" should be understood as embracing halocarbons such as CFCl.sub.3 as well as hydrogen-containing compounds.) In most industrial production processes, a chlorine/fluorine exchange takes place, catalyzed by antimony halide. Antimony pentahalide, particularly antimony pentachloride, is used as the catalyst.
The life of these catalysts is limited. During production of fluorochlorohydrocarbons or fluorohydrocarbons, unwanted halogenated organic compounds, some of which have considerably higher boiling points than the desired products, are also formed in addition to the desired process products. These byproducts dilute the antimony halide catalyst, so that gradually solutions containing antimony halide catalysts and organic impurities and having increasingly diminishing activity form. When the diminishing activity of the catalysts falls below a given limit, the inactive catalyst solution has to be removed from the reactor and be replaced by fresh antimony halide catalyst.
The solutions contain pentavalent and usually also trivalent antimony in the form of the halides, namely predominantly in the form of chlorides, but also to a greater or lesser extent in the form of mixed chlorides/fluorides. Depending on the field of use of the catalyst solution, the fluoride content may also be very low. In addition to the inorganic constituents, halogenated hydrocarbons are also present in the solutions. Which particular halogenated hydrocarbons are present in each case will naturally depend on the respective fluorination process in which the catalyst solution has been used and the manner in which the process is performed.
It is desirable to work up the inactive solutions for economic reasons (recovery of the antimony content), and also particularly for ecological reasons.
Antimony pentahalides and the halogenated hydrocarbons of the inactive solution form a homogenous mixture. Since some of the organic constituents have a boiling point similar to antimony pentachloride, for example, it is not possible to work up the solutions by means of distillation. Due to the formation of homogenous phases and the similar boiling points of antimony halides and impurities, it is difficult to separate the antimony halides from the organic constituents during working up.
Methods for working up inactive solutions containing antimony halide catalysts and organic impurities are already known. One method provides an aqueous work-up procedure. According to the process of British Patent Application No. GB 1,531,799, in order to separate the antimony halide from the organic constituents, sufficient water or aqueous hydrochloric acid is added to the organic solution containing antimony halide catalysts so that two liquid phases form. One phase is formed by the organic constituents, the other, aqueous phase contains the portions of pentavalent antimony. The hydrated antimony compounds can then easily be separated from the organic constituents and reduced if desired. The conversion of the dissolved antimony compounds into anhydrous compounds, in particular into the anhydrous antimony halides, is certainly very involved. Therefore the aqueous antimony trihalide solutions which are obtained after reduction are used directly for producing lead chromate pigments.
Another method provides a non-aqueous work-up procedure. Such a process is described in U.S. Pat. No. 3,760,059. In this procedure an equal volume of trichloroethylene is added to the solution containing antimony halide catalyst and organic impurities, and the mixture is heated in an autoclave. The resulting antimony trichloride, which is crystalline and anhydrous, can be separated from the organic constituents by suction filtering, and if desired, can be processed to produce antimony pentachloride, which then can be re-used as a catalyst. However, the addition of a considerable quantity of trichloroethylene which is necessary in this process (other olefins, apart from perchloroethylene, have proved unusable) leads to a correspondingly higher occurrence of unwanted byproducts having high boiling points.